1. Field of the Invention
The present invention relates to a focus detection device having an infrared filter positioned in a focus detection system for blocking undesired light such as infrared light. The focus detection device can be used in cameras, or similar apparatus.
2. Description of Related Art
FIG. 7 depicts a conventional focus detection device. Light rays pass through different regions of the imaging lens 1 and form an image at a predetermined focusing plane FP after passing through a half transparent mirror 2 and a completely reflective auxiliary mirror 3. The light rays pass through a view field mask 4, a field lens 5, a deflection mirror 6 and a pair of diaphragm masks 7 containing an opening. A pair of secondary imaging lenses 8 form the light rays into images on a pair of photo sensors 9a and 9b located at a secondary focusing plane. The focusing state of the imaging lens 1 can be detected by detecting the difference in phase between the secondary images using the output of the photo sensors. The photo sensors are housed in a single package 9 and produce a focus detection signal as is known. See, for example, U.S. Pat. No. 4,634,255, the disclosure of which is incorporated herein by reference. In addition, the light rays used in focus detection are deflected by a deflection mirror 6. The light rays are deflected such that the secondary imaging lenses 8 and the photo sensor package 9 can be efficiently positioned in the camera body without increasing the camera body size.
The photo sensors typically include photoelectric elements (for example, charge-coupled-devices (CCD)) having a peak spectral sensitivity in the long wavelength region of infrared rays. A filter is necessary to reduce the amount of light in the infrared region to compensate for the relative sensitivity of the human eye. Infrared reduction filters can be divided into two categories, absorption filters and reflective filters. Absorption filters present spatial problems when used in the focus detection module of a camera or similar apparatus. These filters may be thick due to the amount of glass needed to form the base plate, however, a camera is generally small in size. In addition, the wavelength cutting properties of these absorption filters are not sharp. Consequently, reflective infrared reduction filters made from multiple film layers coated on a glass base plate are generally used in the focus detection modules.
The focus detection device disclosed in Japanese Laid-Open Patent Application No. 60-263912 uses a reflective infrared reduction filter 10 (See FIG. 7) positioned between the field lens 5 and the view field mask 4 (i.e., on the front surface of field lens 5). FIG. 4 depicts a focus detection device forming secondary images on the photo sensors 9a and 9b. The projection position of the opening of the diaphragm mask of the focus detection device onto the imaging lens (position I in FIG. 4) is 100-200 mm from the predetermined focusing plane FP. The distance from the predetermined focusing plane FP to the secondary imaging plane on the photo sensors 9a and 9b is at most several dozen millimeters because it is desirable for the focus detection device to be compact. The angle .THETA.1 at the front surface of the field lens 5 is the smallest of the maximum angles of incidence .THETA.1-.THETA.3 made on the imaging plane by the primary light rays at focus detection optical paths 21 and 22.
The transparency of a reflective infrared reduction filter changes dramatically with the angle of incidence. When the filter is arranged at a large angle of incidence, the spectrum distribution of an image deviates from the actual distribution and the intensity of light is diminished. This causes a reduction in detection precision. As a result, it is beneficial to have an arrangement in which the angle of incidence of the light rays is small, preferably not more than 10.degree.. The reflective infrared reduction filter in Japanese Laid-Open Patent Application No. 60-263912 is located at the front surface of the field lens 5 because the smallest angle of incidence is located at the front surface of the field lens 5.
As shown in FIG. 7, there is some separation between the field lens 5 and the predetermined focusing plane FP of the imaging (e.g., photographic) lens. The position of the predetermined focusing plane FP is determined by the position of the auxiliary mirror 3 relative to the film surface. The distance between the predetermined focusing plane FP and the auxiliary mirror 3 should correspond to the distance between the auxiliary mirror 3 and the film surface F. For example, if the auxiliary mirror is moved toward the imaging lens 1 (as shown in dotted lines in FIG. 7), then the predetermined focusing plane FP must move downward while moving in a direction toward the imaging lens 1.
The movement of the auxiliary mirror 3 toward the imaging lens 1 is limited such that the auxiliary mirror 3 will cover all of the light rays necessary for distance measurement used in focus detection. If the mirror is moved too closely to the imaging lens (as shown in dotted lines 3-1 in FIG. 7), then some light rays will not be reflected by the auxiliary mirror 3 leading to an inaccurate distance measurement and improper focusing. The predetermined focusing plane FP is positioned slightly above the bottom B of the mirror box to accommodate some movement of the auxiliary mirror 3. However, it is ideal to have the view field mask 4 in the same position as the predetermined focusing plane FP, but because the focus detection module cannot protrude into the mirror box from the bottom B of the mirror box, there is some separation d between the predetermined focusing plane FP and the field lens 5.
Reflective infrared reduction filters are formed by vaporizing electrically conductive layers of film on a glass base plate. Stress is applied to the glass base plate by the thin film during vaporization. If the glass base plate is too thin, cracks may result from the applied stress during vaporization. Japanese Laid-Open Patent Application No. 4-422 discloses a multi-AF (auto-focus) optical system. Focus detection is performed in a plurality of areas on the screen. However, problems arise when a filter is used with this optical system. FIG. 5 shows the fundamental arrangement of a multi-AF optical system. With this optical system, distance measurements are simultaneously possible at several locations including areas a, b, c and d. This type of focus detection optical system requires a larger field lens 5. When an infrared reduction filter is positioned in front of the field lens 5, the surface area of the filter naturally must be increased as well. Because the strength of a reflective type filter having a large surface area is weaker under the stress applied by the film during vaporization than is a filter having a small surface area, it is necessary to use a thicker glass base plate than in a filter having a small surface area. Accordingly, the separation d referred to above between the predetermined focusing plane FP and the field lens 5 is increased by the thickness of the infrared reduction filter positioned between them. This further separates the field lens 5 from the predetermined focusing plane FP.
Increasing the distance between the field lens 5 and the predetermined focusing plane FP creates problems, discussed in reference to FIG. 6. Assuming the separation d between the predetermined focusing plane FP and the field lens 5 has increased by the amount .delta.1, the light rays 23 and 24 which pass through the boundary of the distance measurement range R on the predetermined focusing plane FP are spread as they pass through the opening of the view field mask 4. The spread of the light rays 23 and 24 when they are incident on the field lens 5 becomes larger as the separation between the predetermined focusing plane FP and the field lens 5 becomes larger. As a result, it becomes necessary to increase the effective surface of the field lens 5 to a similar degree, as shown for example in dotted lines 5-1 in FIG. 6. This increases the size of the focus detection module.
In addition, the light rays 23 and 24 are from different regions on the screen. The field lens 5 must be optically designed with aberration compensation to cope with these different regions. However, lens design becomes difficult as the light rays spread and the spread at the time of incidence on the field lens 5 increases. In particular, when the field lens 5 is in position 5-1 there is a region where the two light rays 23 and 24 overlap (region K indicated by the diagonal lines in FIG. 6). The design of a lens that can cope with the aberrations of the light rays in this region becomes impossible, and the optical properties of the lens become inferior to that of a lens used for cases where the light rays have not spread.
Furthermore, as the separation between the predetermined focusing plane FP and the field lens 5 increases from d to d+.delta.1, it is necessary to also increase the separation L1 between the field lens 5 and the secondary imaging lenses 8 and the separation L2 between the secondary imaging lenses 8 and the photo sensor package 9 to maintain the same secondary imaging magnification and optical properties. The increases in the lengths are denoted as .delta.2 and .delta.3. The total distance from the predetermined focusing plane FP to the photo sensor package 9 becomes .delta.1+.delta.2+.delta.3 longer than the total distance d+L1+L2. This increases the size of the focus detection module.